Improve JvmZipReader handling of CEN magic matches that don't denote an actual CEN entry

Reading backwards, then forwards allows us to skip over the full CEN entry range, which can include CEN magic headers as part of their fields.

Author: Col-E

src/main/java/software/coley/lljzip/format/model/CentralDirectoryFileHeader.java

@@ -36,7 +36,7 @@
  * @author Matt Coley
  */
 public class CentralDirectoryFileHeader extends AbstractZipFileHeader {
-	protected static final long MIN_FIXED_SIZE = 46;
+	public static final long MIN_FIXED_SIZE = 46;
 
 	private transient LocalFileHeader linkedFileHeader;
 

src/main/java/software/coley/lljzip/format/read/JvmZipReader.java

@@ -86,26 +86,40 @@ public void read(@Nonnull ZipArchive zip, @Nonnull MemorySegment data) throws IO
 		end.read(data, endOfCentralDirectoryOffset);
 		zip.addPart(end);
 
-		// Read central directories (going from the back to the front) up until the preceding ZIP file (if any)
-		// but not surpassing the declared cen directory offset in the end of central directory header.
+		// Find the first CEN header.
 		long len = data.byteSize();
-		long centralDirectoryOffset = len - ZipPatterns.CENTRAL_DIRECTORY_FILE_HEADER.length;
-		long maxRelativeOffset = 0;
 		long centralDirectoryOffsetScanEnd = Math.max(Math.max(precedingEndOfCentralDirectory, 0), end.getCentralDirectoryOffset());
-		while (centralDirectoryOffset > centralDirectoryOffsetScanEnd) {
-			centralDirectoryOffset = MemorySegmentUtil.lastIndexOfQuad(data, centralDirectoryOffset - 1L, ZipPatterns.CENTRAL_DIRECTORY_FILE_HEADER_QUAD);
-			if (centralDirectoryOffset >= 0L) {
-				CentralDirectoryFileHeader directory = newCentralDirectoryFileHeader();
-				try {
-					directory.read(data, centralDirectoryOffset);
-				} catch (ZipParseException ex) {
-					// We cannot recover from the CEN reading encountering failures.
-					throw new IOException(ex);
-				}
-				zip.addPart(directory);
-				if (directory.getRelativeOffsetOfLocalHeader() > maxRelativeOffset)
-					maxRelativeOffset = directory.getRelativeOffsetOfLocalHeader();
+		long earliestCentralDirectoryOffset = len - ZipPatterns.CENTRAL_DIRECTORY_FILE_HEADER.length;
+		long maxRelativeOffset = 0;
+		while (true) {
+			// Look backwards for the next CEN magic quad.
+			long offset = MemorySegmentUtil.lastIndexOfQuad(data, earliestCentralDirectoryOffset - 1L, ZipPatterns.CENTRAL_DIRECTORY_FILE_HEADER_QUAD);
+
+			// If the offset is before the END defined CEN offset, then we should stop scanning.
+			// This offset is the first CEN for the given END.
+			if (offset < centralDirectoryOffsetScanEnd)
+				break;
+
+			// Record as the new earliest CEN offset, and loop again.
+			earliestCentralDirectoryOffset = offset;
+		}
+
+		// Read central directories going forward from the first CEN header.
+		// We do this "find the first, then read forward" so that we can skip over data that may
+		// coincidentally hold the CEN magic, but not actually denote the beginning of a central directory entry.
+		long centralDirectoryOffset = earliestCentralDirectoryOffset;
+		while (centralDirectoryOffset >= 0L) {
+			CentralDirectoryFileHeader directory = newCentralDirectoryFileHeader();
+			try {
+				directory.read(data, centralDirectoryOffset);
+			} catch (ZipParseException ex) {
+				// We cannot recover from the CEN reading encountering failures.
+				throw new IOException(ex);
 			}
+			zip.addPart(directory);
+			if (directory.getRelativeOffsetOfLocalHeader() > maxRelativeOffset)
+				maxRelativeOffset = directory.getRelativeOffsetOfLocalHeader();
+			centralDirectoryOffset = MemorySegmentUtil.indexOfQuad(data, centralDirectoryOffset + CentralDirectoryFileHeader.MIN_FIXED_SIZE, ZipPatterns.CENTRAL_DIRECTORY_FILE_HEADER_QUAD);
 		}
 
 		// Determine base offset for computing file header locations with.